FIG. 18 shows a configuration of a related radio communication system.
In FIG. 18, a plurality of radio base stations 10, 11, and 12 that are separated from one another are provided, and each of the radio base stations 10, 11, and 12 covers a predetermined area. The radio base stations 10, 11, and 12 respectively have coverage areas of macro cells MC10, MC11, and MC12, each of which typically having a cell radius of about several km to 10 km. In FIG. 18 the radio base station 10 covers the area of the macro cell MC10, the radio base station 11 covers the area of the macro cell MC11, and the radio base station 12 covers the area of the macro cell MC12.
A radio network controller (RNC: mobile communication control station) 220 controls communication links of the plurality of radio base stations 10, 11, and 12.
The RNC 220 performs radio network control, call processing and the like when the location of a mobile communication terminal 900 is registered, when the mobile communication terminal 900 makes or receives a call, and when the mobile communication terminal 900 performs a handover process.
A core network 230 includes a location register (LR: home memory station) 231, and a gate way (GW: mobile communication network gateway control station) 232, and forms a core dedicated line of a cellular telephone communication.
Now, the macro cell MC11 and the macro cell MC12 are adjacent to the macro cell MC10.
In this case, adjacent cell information is included in system information transmitted from the radio base station 10 to the macro cell MC10.
A primary scramble code (PSC) of a cell may be used as a code to identify the cell. Specifically, the radio base station 10 transmits primary scramble codes (PSCs) of the macro cell MC11 and the macro cell MC12 that are adjacent cells to the coverage area as the adjacent cell list as shown in FIG. 19.
The mobile communication terminal (UE: User Equipment) 900 communicates with a radio base station specifying the macro cell covering its own location by radio to establish a communication link with a communication apparatus of the other party by way of the network 230.
In FIG. 18, the mobile communication terminal 900 is in the area of the macro cell MC10. Thus, the mobile communication terminal establishes the communication with the radio base station 10. When the mobile communication terminal 900 moves to cross the border of macro cells, a radio base station with which the mobile communication terminal 900 communicates is switched according to the movement between cells in order to maintain the communication without cutting the communication link.
For example, as shown in FIG. 18, when the mobile communication terminal moves from the area covered by the macro cell MC10 to the area covered by the macro cell MC11, the base station with which the mobile communication terminal 900 communicates is switched from the radio base station 10 to the radio base station 11.
In this case, the mobile communication terminal searches candidates of neighboring macro cells that can be selected using the PSCs of the adjacent cell list notified in the macro cell MC10 which is the area covering the mobile communication terminal.
A radio base station having high reception intensity is selected among the candidates, and the communication link is established.
Such a handover control is disclosed, for example, in Patent literature 1 (Japanese Unexamined Patent Application Publication No. 2008-271398) and Patent literature 2 (Published Japanese Translation of PCT International Publication for Patent Application, No. 2001-505385).
Incidentally, intensity of radio waves from a radio base station becomes weak inside a building or underground, where communication is limited or unavailable.
In order to deal with such a problem, a small-scale base station that covers an area (cell radius) of about several tens of meters has been proposed as a base station used in a radio communication system.
A cell formed by such a small-scale base station is called a “femtocell”.
FIG. 20 shows a state in which femtocell base stations 15 are installed.
Compared with the macro cell MC10 having a cell radius of about several km to 10 km, the femtocell has an extremely small cell radius.
Further, the femtocell base station 15 is connected to fixed communication lines such as an asymmetric digital subscriber line (ADSL), optical fibers, coaxial cables, through which a communication service of a cellular telephone may be provided.
The base station can be installed in the building in a simple manner by installing the femtocell base station in the building or the like and connecting the femtocell base station to a broadband line that is already installed.
With such a femtocell base station, the number of base stations can be increased in a simple manner and with low cost compared to the case of installing a large macro cell base station. The femtocell base station is a promising technique to further improve the service of a mobile communication network.